Within the wireless test and measurement industry, radio architectures utilizing fixed band filters and technology specific software have traditionally been adequate in supporting the wireless industry. However, the recent acceleration by operators to migrate from 2G/3G to 4G-LTE has created a need for single unit, multi-band, multi-technology scanners. Additionally, new markets, such as military, government, and local municipalities, outside of the traditional commercial wireless arena, are adopting synergistic wireless solutions to commercial carriers but utilize separate and often ad-hoc or dynamic spectrum assignments.
The need for operators and network optimization vendors to have at-will capability to access and deploy all frequencies across many global markets has proven to be challenging, both with funding and the logistics of equipment distribution. Indeed, accomplishing RF frequency agility by traditional means has, to date, proven to be cost and performance prohibitive.
Known receivers used in the industry include a homodyne receiver (zero intermediate frequency (IF)) and a superheterodyne receiver (with IF). The following discussion addresses some of the advantages and disadvantages of these types of known receivers.
Known homodyne receivers include a simple architecture and no high level or direct image issues caused by a local oscillator (LO). However, known homodyne receivers have inherent poor performance of I/Q imbalance, LO leakage, and spurious free dynamic range due to the high requirement of the even order of frequency mixing components. For example, I/Q phase and gain imbalance in known homodyne receivers can cause a different type of image, and known homodyne receivers have terrible LO suppression, poor intermodulation rejection, and are limited in dynamic range. As a result, the use of a homodyne receiver structure does not provide the required dynamic range and does not yield the required performance for an instrument grade receiver.
Known superheterodyne receivers have a good general performance. For example, these structures, when combined with a digital IF, have no I/Q imbalance issue and less of a LO leakage problem. However, in a wide tuning range, image rejection and intermodulation rejection are problematic and spurious signals from mixed components are difficult to control. For example, superheterodyne structures, combined with a digital IF, inherently result in more frequency mixing issues, such as internally generated spurs and mixed byproducts. Considering that such superheterodyne receivers cover several octaves of frequencies, such signal/LO mixing poses a huge threat in the performance of the receiver by inclusion of the unwanted byproducts. Additionally, wide band superheterodyne receivers are often very difficult and costly to manufacture in order to reject the inherent image response and suppress intermodulation non-linearities across wide range front-end filter frequencies.
A large challenge related to the design of a superheterodyne structure comes from its fixed IF. With a fixed IF, the IF filters can be custom-made with surface acoustic wave (SAW) or ceramic technologies. These filters have superior performance in terms of sharp roll-off within the transition band just a few MHz outside of the pass band and thus, are able to reject any incoming unwanted signals from the front-end.
However, fixed IF structures are susceptible to the intermodulation byproducts generated from the overall system that fall within the IF frequency itself. During the design stage, careful overall system frequency planning is required to ensure intermodulation performance. However, this planning often results in unreasonable RF isolation requirements between areas of the overall system and can lead to bulky and costly implementations. The increasing signal bandwidth required by many markets also adds to the difficulty of filter design and IF selection.
In order to reject the image and intermodulation across a wide frequency range, one existing approach for a wide band tuning system utilizing superheterodyne architecture is to use a tunable pre-selector, front-end filter. This approach places a band pass filter that is able to adjust its center to the interested frequency in the very front of the receiver before the first mixer, and the filter is designed to suppress the potential signals that will generate the in-band intermodulation components after the mixer. At a minimum, the frequencies causing image and half-IF responses must be aligned within the tunable filter's stop band across the whole tuning range. However, this filter approach faces a great challenge in design in order to result in good system characteristics over a few octaves of tuning range and accuracy across the expected temperature range. Furthermore, the front-end filter cannot expand the frequency range on an as-needed basis. Indeed, a fixed IF is required when the filter has a sharp roll-off, making the superheterodyne architecture difficult to modify, difficult to integrate, and difficult to migrate.
Another existing approach utilizing a superheterodyne architecture is a more common wide band approach that uses fixed band filtering in the RF front. Instead of using a tunable pre-selector filter, a group of fixed filters are used and switched into the signal path to cover the span of the desired wide band. However, a lot of fixed filters are required, and, because of increasing signal bandwidth and for better intermodulation performance in the IF, each filter requires a custom design leading to bulky and costly high order filters. Furthermore, once a design is finished, the same architecture is not able to expand to different ranges of frequencies.
When considering integration on a single chip, known superheterodyne structures also have inherent limitations in size, integration, and isolation requirements due to the size of the IF filter and the difficulty in designing a wide range tunable filter that covers the whole range of interest. Accordingly, most IC receivers must be designed as a homodyne structure with limited performance.
In view of the above, there is a need for an improved wide tuning range receiver.